Al—Mg-base aluminum alloys have been known to express a superplastic phenomenon that shows an elongation of as high as 300% at a strain rate of about 10−3/sec in a high temperature region. The aluminum alloy is molded into an arbitrary shape, for example so as to follow a die having an arbitrary shape, by taking advantage of this feature by a gas pressure after heating an aluminum alloy sheet at a high temperature. In a known art on an aluminum alloy sheets for superplastic molding, a molded article having a complex shape, which is difficult to produce by conventional press molding at room temperature, may be obtained by using the aluminum alloy sheet (for example, see Japanese Patent Publication No. 2,831,157).
More recently, an art on high-temperature/high-speed and molding that largely enhances productivity has been disclosed, wherein the strain rate has been increased one digit or more, for example in the range from 10−2 to 1/sec (for example, see JP-A-8-199272 (“JP-A” means unexamined published Japanese patent application), JP-A-10-259441, JP-A-2003-342665, JP-A-2004-225114 and JP-A-2004-285390, and Japanese Patent Publication No. 3,145,904).
In high temperature molding applied in a region where the strain rate is high in recent years, crystal structures are controlled during high-temperature/high speed molding in order to secure better high-temperature/high-speed moldability. For example, JP-A-8-199272 discloses an art for preventing crystal grains from being abnormally grown during high temperature molding by adding an appropriate amount of one or plural elements of Mn, Cr, Zr, V, Ti and B.
JP-A-10-259441 discloses an art for fining recrystallized grains in recrystallization of an alloy during high-temperature deformation by adding an appropriate amount of one or plural elements of Mn, Cr and Zr. JP-A-2003-342665 further discloses an art for giving good moldability and good appearance after molding while enhancing the strength after molding by stabilizing recrystallization that occurs during high-temperature deformation by adding an appropriate amount of one or both of Mn and Cr.
The crystal structure has been controlled in the conventional art of high-temperature/high speed molding by adding an appropriate amounts of various transition elements represented by Mn and Cr.
Slip at crystal grain boundaries is a major mechanism of deformation in superplastic molding in which an Al—Mg-base aluminum alloy is molded at a strain rate of 10−3/s, and it has been known that elongation by superplastic molding is larger as crystal grains of the material before molding are finer. In Japanese Patent Publication No. 2,831,157 on superplastic molding, for example, the average crystal grain size is prescribed to be 20 μm or less for securing high superplastic moldability.
On the other hand, subgrains are formed in the crystal grain that constitutes the aluminum alloy in high-temperature/high speed molding applied in a strain rate region as high as from 10−2 to 1/s during molding. The term “subgrain” as used herein refers to grains composed of grain boundaries (referred to subgrain boundaries) with a grain boundary angles of less than 15° within usual crystal grains composed of crystal grain boundaries with a grain boundary angle (the difference of orientation between adjoining grains) of 15° or more. The subgrain structure formed during molding seems to strongly affect high-temperature/high-speed moldability and the strength of the molded articles after molding. However, it has not been examined in conventional Al—Mg-base aluminum alloys what is the configuration of the optimum subgrain structures for high-temperature/high-speed moldability. For example, JP-A-8-199272 prescribes the average crystal grain diameter of the material before molding to be in the range from 15 to 120 μm, and discloses to add appropriate amounts of Mn, Cr, Zr and the like in order to prevent crystal grains from being abnormally grown by high-temperature/high-speed molding. However, the patent only describes the crystal grain structure, and no examination on the subgrain structure is disclosed. Likewise, JP-A-10-259441 prescribes the average crystal grain diameter in the range from 20 to 200 μm, and discloses to add appropriate amounts of Mn, Cr, Zr and the like for fining crystal grains by recrystallization of the alloy during high-temperature deformation. However, these descriptions are only related to the crystal grains.
Furthermore, the deformation mechanism of the Al—Mg-base aluminum alloy in high-temperature/high speed molding has not been sufficiently elucidated, and it remains unknown what crystal structure is most suitable and what method for controlling the structure is necessary in order to obtain high-temperature/high-speed moldability and in order to enhance the strength of the molded article. Consequently, troubles that have been often encountered in the production site for producing the molded article by taking advantage of the high-temperature/high-speed molding art include breakage of the material in the course of molding or insufficient strength of the product after molding depending on the conditions such as molding temperatures and strain rate.
When excess amounts of Mn and Cr are added for the above-mentioned objects, Cr-base giant intermetallic compounds may be formed during melt-casting, and moldability has been often deteriorated by mingling of the giant intermetallic compound since the Cr-base intermetallic compound may serve as an origin of breakage. This molded article cannot be used for a member, for example a member of a transport machine, that suffers from repeated loads since fatigue characteristics of the molded article obtained are largely decreased, even when high-temperature/high speed molding was successful.